Thanks everyone for the advices. So it seems like a good copy and a bad copy can only be determined when you pixel peep? If Im not a professional and only take pictures of my baby then it doesnt really matter?

It is not that straight forward to verify an AF zoom lens and compare two copies.

Decentering check is easy - the 'slanted horizon', followed by the 'upside down - same image', would show if one side is different from the other. (Note - it takes four images - two angles for the horizon). See Jerome Munich pic below.

If the lens is not decentered, you worry about its electronics and its lens alignment. In most lenses, there are tight tolerances, but they should all adhere to a similar quality check.

Comparing the two lenses requires fine precision of the zoom control and of the focus control. Even metering can be tricky. If anyone of the parameters is different, the image will not match at the pixel peaking level, and you'd be hard pressed to decide which one is the better one.

And then you really should use a tripod, otherwise you'll include all of the operator behavior in the lens quality analysis as well.

A lens that has a problem, which is not uncovered by the decentering test, will usually reveal this fairly quickly as you take some images.

The news-papers on wall test is probably the quickest method to verify sharpness across the frame, both wide open and stopped down, at different zoom levels. Doing the test correct takes some diligence, and consistency is very key here.

Plus, you should try different distances to the wall. Lenses have some level of field curvature that may be very pronounced at close distances, but non-existent at far distances, or a mix thereof.

But if you have to pixel peep to find any difference at all, you can assure yourself that the two lenses are nearly identical, or that both met the QA criteria.